Aqua regia and hydrogen peroxide HCl combination to remove Ni and NiPt residues

ABSTRACT

A method for cleaning residues from a semiconductor substrate during a nickel platinum silicidation process is disclosed, including a multi-step residue cleaning, including exposing the substrate to an aqua regia solution, followed by an exposure to a solution having hydrochloric acid and hydrogen peroxide. The SC2 solution can further react with remaining platinum residues, rendering it more soluble in an aqueous solution and thereby dissolving it from the surface of the substrate.

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 13/094,967filed on Apr. 27, 2011, entitled “COMPOSITION AND METHOD TO REMOVEEXCESS MATERIAL DURING MANUFACTURING OF SEMICONDUCTOR DEVICES”, to U.S.application Ser. No. 13/276,973 filed on Oct. 19, 2011, entitled “METHODFOR CLEANING PLATINUM RESIDUES ON A SEMICONDUCTOR SUBSTRATE”, to U.S.application Ser. No. 13/296,444 filed on Nov. 14, 2011, entitled“PROCESS TO REMOVE Ni AND Pt RESIDUES FOR NiPtSi APPLICATIONS”, to U.S.application Ser. No. 13/292,906 filed on Nov. 9, 2011, entitled “PROCESSTO REMOVE Ni AND Pt RESIDUES FOR NiPtSi APPLICATIONS USING AQUA REGIAWITH MICROWAVE ASSISTED HEATING”, and to U.S. application Ser. No.13/295,333 filed on Nov. 14, 2011, entitled “PROCESS TO REMOVE Ni AND PtRESIDUES FOR NiPtSi APPLICATIONS USING CHLORINE GAS”, the disclosures ofwhich are incorporated by reference herein.

TECHNICAL FIELD

Provided are methods of forming silicides, and more particularly tomethods of removing nickel and platinum residues.

BACKGROUND OF THE INVENTION

Silicide processes have been used to improve the conductivity ofpolysilicon gate and source/drain regions at the transistor level of anintegrated circuit. The silicide layer provides a good ohmic contact atthe interface of the gate and source/drain electrodes and the metalinterconnects, reducing the contact resistance of the electrodes. Thesilicide materials have been changed from titanium silicide at above 130nm device dimensions, to cobalt silicide at between 90 nm to 130 nmdevice dimensions, to nickel silicide at between 65 nm to 90 nm devicedimensions, and now to nickel platinum silicide for device dimensionsbelow 65 nm.

Advanced semiconductor fabrication processes currently use nickel andnickel alloy silicide due to their low electrical resistivity, lowsilicon consumption, good resistance behavior in narrow lines, and lowprocessing temperature. A conventional method of forming a nickelsilicide includes depositing a nickel layer on a semiconductor wafer,followed by a first rapid thermal process (RTP) at low temperatures ofabout 300 C to react nickel with silicon to produce high resistancenickel silicide phase Ni₂Si or NiSi. A selective etching process isperformed to remove the unreacted nickel layer, and a second RTP athigher temperatures of about 450 C is performed to convert highresistance nickel silicide phase Ni₂Si or NiSi to low resistance nickelsilicide phase NiSi₂.

A challenge of nickel silicide is the potential spiking effect, togetherwith possible lateral diffusion to the channel region. Thus nickel alloysilicide, especially nickel platinum silicide, has been used to improvethe thermal stability of nickel silicide. For example, nickel platinumsilicide with 5 to 10 atomic percent (at %) platinum content canincrease the silicide nucleation temperature to 900 C and theagglomeration temperature to 750 C, while still retaining the sameconductivity as that of pure nickel silicide. However, platinum isdifficult to etch, resulting in potential platinum residue issues duringthe removal of the unreacted metal layer.

Therefore, what is needed are etch solutions and methods that allow forthe effective removal of advanced materials (e.g., nickel and platinum)during semiconductor processing and manufacturing.

SUMMARY OF THE DESCRIPTION

In some embodiments, the present invention discloses a cleaning processto remove nickel and platinum residues using a combination of chemicalsthat can react to generate chlorine-containing gas either in gaseousform or dissolved in a fluid. The chlorine-containing gas can convertplatinum to a more soluble state which can facilitate the residueremoval.

In some embodiments, the present invention discloses a cleaning processusing a sequential application of dilute aqua regia and a mixture ofhydrochloric acid and hydrogen peroxide. The dilute aqua regia canremove nickel and convert platinum to a soluble state. The mixture ofhydrochloric acid and hydrogen peroxide can react to generate dissolvedchlorine gas, which can remove nickel, platinum and nickel/platinumresidues that still remain after the aqua regia cleaning process. Forexample, dissolved chlorine gas generated in the mixture can furtheroxidize the soluble chloroplatinous acid (H₂PtCl₄) to chloroplatinicacid (H₂PtCl₆), which is one of the most soluble platinum forms of Pt⁴⁺.

In some embodiments, the present invention discloses methods to formnickel platinum silicide, comprising depositing a layer of nickelplatinum on a silicon substrate, or a substrate comprising at least asilicon or germanium region. After performing a first rapid thermalprocess to react nickel and platinum with silicon to form nickelplatinum silicide, the substrate is exposed to a first solution ofdilute nitric acid. The dilute nitric acid can remove the unreactednickel and platinum. After performing a second rapid thermal process toconvert nickel platinum silicide to low resistant nickel platinumsilicide phase, the substrate is exposed to a second solution of diluteaqua regia to further remove the remaining nickel and platinum. Thesubstrate is then exposed to a mixture of hydrochloric acid and hydrogenperoxide to further remove the remaining platinum residues.

In some embodiments, the present invention discloses methods to form asemiconductor device, comprising forming a transistor structure on asemiconductor substrate and forming a nickel platinum silicide on atleast a gate electrode or a source/drain region of the transistorstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The drawings are not to scale and the relative dimensionsof various elements in the drawings are depicted schematically and notnecessarily to scale.

The techniques of the present invention can readily be understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIGS. 1A-1E illustrate an illustrative process flow for forming asemiconductor device according to some embodiments of the presentinvention.

FIG. 2 illustrates an illustrative process flowchart for cleaning asurface according to some embodiments of the present invention.

FIG. 3 illustrates an illustrative process flowchart for forming anickel platinum silicide according to some embodiments of the presentinvention.

FIG. 4 illustrates an illustrative process flowchart for forming asemiconductor device according to some embodiments of the presentinvention.

DETAILED DESCRIPTION

A detailed description of one or more embodiments is provided belowalong with accompanying figures. The detailed description is provided inconnection with such embodiments, but is not limited to any particularexample. The scope is limited only by the claims and numerousalternatives, modifications, and equivalents are encompassed. Numerousspecific details are set forth in the following description in order toprovide a thorough understanding. These details are provided for thepurpose of example and the described techniques may be practicedaccording to the claims without some or all of these specific details.For the purpose of clarity, technical material that is known in thetechnical fields related to the embodiments has not been described indetail to avoid unnecessarily obscuring the description.

In some embodiments, the present invention discloses a process to removenickel and platinum residues on a semiconductor surface using acombination of chemicals that can react to generate chlorine-containinggas either in gaseous form or dissolved in a fluid. Thechlorine-containing gas can convert platinum to a more soluble statewhich can facilitate the residue removal. For example, a mixture ofhydrochloric acid and hydrogen peroxide (commonly called Standard Clean2 or SC2 in semiconductor industry) can generate chlorine gas during themixing.

Aqua regia (etching solution comprising HNO₃+HCl) can be used to etchplatinum in unreacted nickel platinum metal layer. Platinum metal can beoxidized by the oxidant to form platinum ions, which are then reactedwith chloride ions to form soluble hexachloroplatinic acid.

Generally, platinum solid can be dissolved, by reacting with nitric acidto form platinum ions Pt⁴⁺,3Pt(s)+4NO₃ ⁻(aq)+16H⁺(aq)→3Pt⁴⁺(aq)+4NO(g)+8H₂O(l)

The oxidized platinum ions then react with chlorine ions Cl⁻ to formsoluble chloroplatinate ions PtCl₆ ²⁻, which is one of the most solubleforms of platinum ions:Pt⁴⁺(aq)+6Cl⁻(aq)→PtCl₆ ²⁻(aq)

However, the reaction of platinum with aqua regia is considerably morecomplex. For example, mixed phases of nickel platinum silicide can beformed at various RTP temperatures, with lower RTP temperaturesresulting in a higher proportion of metal-rich silicide phases, whichare less susceptible to be removed by aqua regia. Further, aqua regia isknown to degrade nickel platinum silicide quality, especially for twostep thermal processes.

In general, chlorine ions can react with platinum ions to form differentchloro-platinum compounds with different degrees of solubility. Forexample, platinum reacting with aqua regia can additionally produce theless soluble chloroplatinous acid H₂PtCl₄:Pt(s)+2HNO₃(aq)+4HCl(aq)→(NO)₂PtCl₄(s)+3H₂O(l)+½O₂(g)(NO)₂PtCl₄(s)+2HCl(aq)→H₂PtCl₄(aq)+2NOCl(g)

Thus cleaning platinum residues with aqua regia might be incomplete,leaving platinum residues on the semiconductor surface.

In some embodiments, the present invention discloses exposing a nickelplatinum surface to a gaseous chlorine to provide full dissolution ofplatinum, for example, by converting the less soluble chloroplatinousacid H₂PtCl₄ to the more soluble hexachloroplatinic acid H₂PtCl₆:H₂PtCl₄(aq)+Cl₂(g)→H₂PtCl₆(aq)

In some embodiments, the present invention discloses exposing a nickelplatinum surface to a mixture of chemicals that can generate gaseouschlorine. In some embodiments, the present invention discloses exposingthe surface to a mixture of hydrogen peroxide and hydrochloric acid,which can form chlorine during the mixing:H₂O₂(aq)+HCl(aq)→Cl₂(g)+2H₂O(aq)

In some embodiments, the mixture solution is heated to a temperatureless than about 80 C, for example, between about 60 and about 80 C.

In some embodiments, the present invention discloses a process to removenickel and platinum residues from a nickel platinum surface of asubstrate by exposing the surface to a mixture of hydrogen peroxide andhydrochloric acid, which is commonly called SC2 or HPM (hydrochloricacid-hydrogen peroxide-water mixture). In some embodiments, the nickelplatinum surface can be first exposed to aqua regia, e.g., a solutioncomprising a mixture of nitric acid and hydrochloric acid. The aquaregia can remove nickel and platinum residues from the surface. In someembodiments, the nickel platinum surface can be additionally oralternatively exposed to a dilute nitric acid solution, a sulfuric acidsolution, or a sulfuric peroxide solution (comprising a mixture ofsulfuric acid and hydrogen peroxide) to remove nickel and some portionof platinum. For example, the nickel platinum surface can be exposed toa sequence of a sulfuric peroxide solution followed by an aqua regiasolution; or a dilute nitric acid followed by an aqua regia solution.Afterward, the nickel platinum surface can be exposed to a mixture ofhydrogen peroxide and hydrochloric acid, which can remove the remainingresidues.

In some embodiments, the present invention discloses a process to removenickel and platinum residues using dilute aqua regia solution and amixture of hydrogen peroxide and hydrochloric acid. In some embodiments,the aqua regia solution and the a mixture of hydrogen peroxide andhydrochloric acid are applied in sequence after thermal processing,where the aqua regia etches nickel and platinum, and the a mixture ofhydrogen peroxide and hydrochloric acid further dissolves remainingplatinum.

In some embodiments, the present invention discloses a nickel andplatinum cleaning process using a two step cleaning process where thefirst cleaning step etches platinum, and the second cleaning step cleansany remaining platinum not etched by the first cleaning step. Forexample, the first cleaning step can be performed after a rapid thermalprocess, reacting nickel and platinum with silicon to form nickelplatinum silicide. In addition the first cleaning steps can be tailoredto convert the platinum to a more soluble state which can facilitate thesecond cleaning step.

In some embodiments, the first cleaning step includes exposing thesubstrate to an aqua regia solution to etch nickel. Aqua regia alsoincludes nitric acid, which is an oxidant acid, and thus can partiallyoxidize platinum, for example, to platinum Pt⁴⁺, which has highsolubility in hydrochloric acid. The remaining platinum that has notbeen dissolved in aqua regia is then subjected to the second cleaningstep using a mixture of hydrogen peroxide and hydrochloric acid todissolve the remaining platinum.

In some embodiments, the present invention discloses a process ofremoving nickel and platinum residues using dilute nitric acid, diluteaqua regia, and a mixture of hydrogen peroxide and hydrochloric acid. Insome embodiments, the dilute nitric acid, dilute aqua regia, and mixtureof hydrogen peroxide and hydrochloric acid are applied in sequence afterthermal processing, where the dilute nitric acid mainly etches nickel,the aqua regia mainly dissolves platinum, and the mixture of hydrogenperoxide and hydrochloric acid further dissolves remaining platinum.

In some embodiments, the separation of etching materials, e.g., nickeletching in a first dilute nitric acid and platinum etching in a secondaqua regia solution, can be effective in removing nickel and platinumresidue while preventing damage to the substrate. For example, platinumis unreactive and generally needs to be oxidized to platinum ions, whichthen can be dissolved in chlorine-based chemistries, such as an aquaregia solution to form soluble hexachloroplatinic acid (H₂PtCl₆).However, platinum can be oxidized to various oxidation states, such asPt⁺, Pt³⁺, or Pt⁴⁺, each with different solubility in chlorine-basedchemistries. For example, the Pt⁺ and Pt³⁺ oxidation states are harderto dissolve than the Pt⁴⁺ oxidation state, and thus directly dissolvingmultiple oxidation states of platinum ions in aqua regia may need highconcentration of hydrochloric acid, such as a typical aqua regiasolution of about 30 wt % hydrochloric acid and about 14 wt % nitricacid. The high concentration aqua regia, though effective in dissolvingplatinum, can also cause damage to the device. Alternatively, platinumcan be dissolved in strong oxidant chemistries, such as a sulfuricperoxide mixture, but the strong oxidant etching of platinum tends to beincomplete, leaving platinum residues. In the present specification, theterms “platinum etch”, “platinum removal”, or “platinum dissolving” havesimilar meaning, which is to remove platinum from the surface of thesubstrate. Further, the present specification discloses a mixture ofhydrogen peroxide and hydrochloric acid, but the invention is not solimited, and any mixture of chemicals that can generate chlorine uponmixing can be used.

In some embodiments, the present invention discloses a nickel andplatinum cleaning process using a three step cleaning process where thefirst cleaning step mainly etches nickel, the second cleaning stepmainly etches platinum, and the third cleaning step cleans any remainingplatinum. For example, the first and second cleaning steps can beperformed after a rapid thermal process, reacting nickel and platinumwith silicon to form nickel platinum silicide. In addition the first andsecond cleaning steps can be tailored to convert the platinum to a moresoluble state which can facilitate the third cleaning step.

In some embodiments, the first cleaning step comprises an acid, such asdilute nitric acid, to etch nickel. Nitric acid is also an oxidant acid,and thus can partially oxidize platinum, for example, to platinum Pt⁴⁺,which has high solubility in hydrochloric acid. The high solubilityplatinum is then subjected to the second cleaning step using an aquaregia solution, dissolving the platinum. With some platinum alreadyconverted to Pt⁴⁺ by the nitric acid in the first step, the aqua regiasolution can be more dilute and still effective at removing platinum. Insome embodiments, the dilute aqua regia can be more dilute than aconventional aqua regia solution, for example, up to 2× more dilute innitric acid and hydrochloric acid.

In some embodiments, the present invention discloses a method forcleaning residues from a surface of a substrate, including exposing thesurface, after a rapid thermal process, to a first solution containing amixture of nitric acid and hydrochloric acid. Subsequently, the surfaceis exposed to a second solution containing dissolved chlorine gas.

In some embodiments, the present invention discloses a method forcleaning residues from a surface of a substrate, including exposing thesurface, after a first rapid thermal process, to a first solutioncomprising dilute nitric acid, wherein the first solution does notcomprise a chlorine-based acid. Subsequently, the surface is exposed,after a second rapid thermal process, to a second solution containing amixture of nitric acid and hydrochloric acid. Afterward, the surface isexposed to a third solution containing a mixture of hydrogen peroxideand hydrochloric acid.

In some embodiments, the mixture of hydrogen peroxide and hydrochloricacid is heated to a temperature of about 80 C, for example to betweenabout 60 C and about 100 C.

In some embodiments, the concentration of the dilute nitric acid in thefirst cleaning solution is less than about 20 wt %, and morespecifically less than about 10 wt %. The concentration of dilute nitricacid may be greater than about 2 or about 3 wt %. In some embodiments,the concentration of the nitric acid in dilute aqua regia is less thanabout 15 wt % and more specifically less than about 10 wt % and theconcentration of the hydrochloric acid in dilute aqua regia is less thanabout 20 wt % and more specifically less than about 16 wt %. Forexample, the dilute aqua regia can include less than about 10 wt %nitric acid and less than about 20 wt % hydrochloric acid.Alternatively, the dilute aqua regia can include less than about 15 wt %nitric acid with less than about 16 wt % hydrochloric acid.

In some embodiments, the composition of the solution is about 1:1:5 orabout 1:1:6, which corresponds one part of hydrogen peroxide, one partof hydrochloric acid, and 5 or 6 parts of water. The concentration ofhydrogen peroxide in the mixture can be less than or equal to about 20wt % or 25 wt %. The concentration of hydrochloric acid in the mixturecan be less than or equal to about 20 wt % or 25 wt %.

In some embodiments, the cleaning process is performed for less thanabout 10 minutes, more specifically for about 5 minutes or less. Thetemperature of the solution, e.g., the dilute nitric acid, the diluteaqua regia, or the mixture of hydrogen peroxide and hydrochloric acid,can be less than or equal to about 80 C or less than about 60 C. In someembodiments, a water rinse is performed after the cleaning steps. Forexample, the surface is rinsed with water after being exposed to dilutenitric acid, after being exposed to dilute aqua regia solution, or afterbeing exposed to the mixture of hydrogen peroxide and hydrochloric acid.

In some embodiments, the cleaning solution of dilute nitric acid in thefirst cleaning step does not include chlorine-based acid. This solutioncan include a non-chlorine chemical, such as a solvent, an acidcomprising fluorine (e.g., HF), or an organic acid (e.g., acetic acid orcarbonyl acid).

In some embodiments, the present invention discloses methods to formnickel platinum silicide, using a two step cleaning process to removenickel and platinum residues. The first cleaning step mainly etchesnickel and platinum, including a first solution containing an aqua regiasolution, with the nitric acid component in the aqua regia solutionoxidizing the platinum residues and the hydrochloric acid element in theaqua regia solution reacting with the oxidized platinum to form variousforms of soluble chloro-platinum acid. The second step cleans remainingplatinum residues, including a second solution containing a mixture ofhydrogen peroxide and hydrochloric acid.

In some embodiments, the present invention discloses methods to form anickel platinum silicide, including providing a substrate having atleast one silicon-containing region. For example, the silicon-containingregion can be a polysilicon gate electrode, or a silicon germaniumsource or drain region. A layer including nickel and platinum is thenformed on the substrate, for example, by a physical vapor deposition(PVD) process. A thermal process, e.g., a rapid thermal process (RTP),may be performed on the substrate including the nickel platinum layer,reacting the nickel and platinum with the silicon in the substrate toform nickel platinum silicide. The substrate is then exposed to a firstcleaning solution including aqua regia containing dilute nitric acid andhydrochloric acid, for example, to remove nickel and platinum residuesfrom the nickel platinum layer. Afterward, the substrate is exposed to amixture of hydrogen peroxide and hydrochloric acid, cleaning anyremaining residues of platinum.

In some embodiments, the present methods involve other featuresdescribed above, such as composition of the mixture of hydrogen peroxideand hydrochloric acid, temperature, concentration, and cleaning time.

In some embodiments, the rapid thermal process is performed attemperature between about 300 C to about 450 C for less than about 1minute in nitrogen environment.

In some embodiments, the present invention discloses methods to formnickel platinum silicide, using a three step cleaning process to removenickel and platinum residues. The first cleaning step mainly etchesnickel, comprising a first solution comprising nitric acid without anychlorine-based acid (e.g., without hydrochloric acid). For example, thefirst solution can be dilute nitric acid. Alternatively, the firstsolution can be a mixture of nitric acid with a non-chlorine-based acidor non-chlorine-based chemical, such as a solvent, an acid comprisingfluoride (e.g., HF), or an organic acid (e.g., acetic acid or carbonylacid). The dilute nitric acid solution can also oxidize a portion of theplatinum residues, rendering them easier to dissolve in the secondcleaning solution. The process conditions of the first cleaning step canbe optimized to facilitate the subsequent platinum removal process, forexample, to oxidize platinum to Pt⁴⁺, which can be easily reacted withchlorine-based chemistries to form soluble platinum compounds, insteadof Pt⁺ or Pt³⁺, which can be harder to remove in the second cleaningstep using dilute aqua regia. Further, the dilute nitric acid solutioncan be tailored to prevent damage to the substrate, for example, byemploying cleaning solutions that are not designed to aggressivelyremove platinum.

The second cleaning step mainly removes platinum. For example, thesecond solution in the second step can comprise an aqua regia solution,with the nitric acid component in the aqua regia solution oxidizing theplatinum residues and the hydrochloric acid element in the aqua regiasolution reacting with the platinum to form soluble hexachloroplatinicacid. Further, the second cleaning step can be assisted by the oxidizingaction of platinum in the first cleaning step through the dilute nitricacid. Therefore, the aqua regia solution can be made more dilute, forexample, as compared to typical prior art aqua regia platinum etching of14 wt % nitric acid and 30 wt % hydrochloric acid.

The third cleaning step mainly removes the remaining platinum after thefirst two cleaning steps. For example, the third solution in the thirdcleaning step can use a mixture of hydrogen peroxide and hydrochloricacid, which can generate chlorine gas to convert the less solublechloroplatinous acid to the more soluble chloroplatinic acid.

In some embodiments, the present invention discloses methods to form anickel platinum silicide, involving providing a substrate having atleast one silicon-containing region. For example, the silicon-containingregion can be a polysilicon gate electrode, or a silicon germaniumsource or drain region. A layer including nickel and platinum is thenformed on the substrate, for example, by a physical vapor deposition(PVD) process. A first thermal process, preferably a rapid thermalprocess (RTP), is performed on the substrate comprising the nickelplatinum layer, reacting the nickel and platinum with the silicon in thesubstrate to form various phases of nickel platinum silicide. Thesubstrate is then exposed to a first cleaning solution comprising dilutenitric acid, for example, to remove unreacted nickel (and/or platinum)from the nickel platinum layer. As disclosed above, the dilute nitricacid solution mainly removes nickel and partially oxidizes platinum. Insome embodiments, some platinum can be dissolved or removed. A secondthermal process, also preferably a rapid thermal process, is performedon the substrate comprising the nickel platinum silicide, to furtherreact the various phases of nickel platinum silicide with silicon in thesubstrate to form low resistive nickel platinum silicide. The substrateis then exposed to a second cleaning solution comprising aqua regiacomprising dilute nitric acid and hydrochloric acid, for example, toremove platinum residues from the remaining nickel platinum layer. Forexample, the nitric acid can oxidize platinum, and the hydrochloric acidcan dissolve the oxidized platinum. As disclosed above, the present aquaregia solution can be more dilute, but still effective, than typicalaqua regia used in cleaning nickel platinum residues, perhaps due to theprior oxidation of platinum to Pt⁴⁺ by the first cleaning step usingdilute nitric acid. Afterward, the substrate is exposed to a mixture ofhydrogen peroxide and hydrochloric acid, cleaning any remaining residuesof platinum.

In some embodiments, the present methods involves other featuresdescribed above such as composition of the mixture of hydrogen peroxideand hydrochloric acid, temperature, concentration, and cleaning time.

In some embodiments, the first rapid thermal process is performed attemperature below about 380 C, for example, at about 300 C for less than1 minute in nitrogen ambient. In some embodiments, the second rapidthermal process is performed at temperature above about 300 C, forexample, at about 450 C for less than 1 minute in nitrogen ambient.

In some embodiments, the cleaning solution of dilute nitric acid in thefirst cleaning step does not include chlorine-based acid. The solutioncan include a non-chlorine chemical, such as a solvent, an acidcomprising fluorine (e.g., HF), or an organic acid (e.g., acetic acid orcarbonyl acid).

In some embodiments, the present invention discloses methods to form asemiconductor device, involving forming at least a nickel platinumsilicide contact for a transistor structure. The transistor structurecan comprise a gate electrode and a source/drain region over asemiconductor substrate. The gate electrode can include an exposed dopedpolysilicon layer. The source/drain region can include an exposedsilicon or a silicon germanium area. A layer including nickel andplatinum is then formed on the substrate, for example, by a physicalvapor deposition process. The nickel platinum layer can be deposited onthe exposed portions of the gate electrode and source/drain region,covering the gate electrode or the source/drain region.

An optional first rapid thermal process is performed on the substrate,reacting the nickel and platinum with the silicon in the gate electrodeor the source/drain region to form various phases of nickel platinumsilicide, such as nickel rich silicide. The substrate is then optionallyexposed to a first cleaning solution comprising dilute nitric acid, forexample, to remove unreacted nickel (and/or platinum) from the nickelplatinum layer. A second thermal process, also preferably a rapidthermal process, is performed on the substrate including the nickelplatinum silicide, to further react the various phases of nickelplatinum silicide with silicon in the substrate to form low resistivenickel platinum silicide. The substrate is then exposed to a secondcleaning solution including aqua regia containing dilute nitric acid andhydrochloric acid, for example, to remove platinum residues (includingPt or NiPt residues) from the remaining nickel platinum layer.Afterward, the substrate is exposed to a mixture of hydrogen peroxideand hydrochloric acid, cleaning any remaining residues of platinum.

In some embodiments, the present methods involve other featuresdescribed above, such as composition of the mixture of hydrogen peroxideand hydrochloric acid, temperature, concentration, cleaning time, andrapid thermal process conditions.

In some embodiments, the cleaning solution of dilute nitric acid in thefirst cleaning step does not include chlorine-based acid. The solutioncan include a non-chlorine chemical, such as a solvent, an acidcomprising fluorine (e.g., HF), or an organic acid (e.g., acetic acid orcarbonyl acid).

In some embodiments, the post silicidation clean process is selectivetowards other exposed materials such as Si₃N₄ (spacers), SiO₂ (fieldoxide), NiPtSi (contact electrodes), and NiPtSiGe (contact electrodes ofsource/drain for strain applications).

FIGS. 1A-1E illustrate an illustrative process flow for forming asemiconductor device according to some embodiments of the presentinvention. In FIG. 1A, a transistor structure 100 is formed on asubstrate 110, comprising isolation regions 150 to isolate theneighboring devices, source and drain regions 140A and 140B sandwichinga gate electrode 120 including a gate dielectric 125 and a gateconductor 122. Spacers 130 cover the sidewalls of the gate electrode120. The substrate 110 can be a semiconductor substrate, or anysubstrates having a layer of semiconductor material. For example, thesubstrate 110 can be a single crystal silicon substrate. The substrate110 can be a silicon-germanium substrate, or can have a silicongermanium layer disposed on top. The gate conductor 122 can comprisedoped polysilicon. The top surfaces of the gate electrode 120 and thesource and drain regions 140A and 140B may be exposed. FIG. 1A shows anexample of a metal-oxide-semiconductor field effect transistor (MOSFET)structure 100, but the invention is not so limited, and can include anytransistor structure, such as bipolar transistors, fin transistors ordouble gate transistors. In addition, the present process flow describesa silicidation process for gate electrode 120 and on source and drainregions 140A and 140B, but the invention is not so limited, and caninclude silicidation for any combination, for example, for only for thegate electrode 120, or only for the source or drain regions 140A or140B.

In FIG. 1B, a layer 160 including nickel and platinum is formed on thetransistor structure, covering the exposed surfaces of the gateelectrode and the source and drain regions. The nickel platinum layer160 may be deposited using PVD process. The platinum concentration inthe layer can be between about 5 wt % and about 15 wt %, for exampleabout 10 wt %. Additional surface preparation can be performed, such asa preclean step with dilute hydrofluoric acid and/or a native oxideremoval step for the exposed gate electrode and source/drain regions.

In FIG. 1C, the substrate, together with the transistor structure 100and the nickel platinum layer 160 is annealed, for example, by a firstrapid thermal process, to react nickel and platinum in the nickelplatinum layer 160 with the silicon in the gate electrode 120 andsource/drain regions 140A/140B. A nickel platinum silicide layer 162 isformed at the top surface of the gate electrode 120, and nickel platinumsilicide layers 164 are formed at the top surface of the source/drainregions. Unreacted nickel platinum layer 166 remains in areas protectedby the spacers and the isolation regions 150, and can include excessnickel platinum remaining after the rapid thermal reaction. The firstrapid thermal process can involve an anneal in nitrogen environment, attemperature lower than about 380 C for less than a minute. For example,a preferred rapid thermal process comprises annealing at about 300 C forabout 30 seconds. The first rapid thermal process can react nickel andplatinum with silicon to form various phases of nickel platinumsilicide, for example, high resistive phases of mono-nickel platinumsilicide or di-nickel platinum silicide.

In FIG. 1D, the substrate surface, including unreacted nickel platinumlayer 166, together with nickel platinum silicide layers 162, 164 at thegate electrode and the source/drain regions, are exposed to a firstcleaning solution comprising dilute nitric acid. In some embodiments,the first cleaning solution contains only dilute nitric acid, i.e.,nitric acid diluted in water. In some embodiments, the first cleaningsolution comprises dilute nitric acid and a non-chlorine based acid. Theexclusion of a chlorine based acid, such as hydrochloric acid, can serveto reduce potential damage caused by the chlorine based acid after thefirst rapid thermal process. For example, the first solution can includea solvent, an acid containing fluorine, or an organic acid (e.g., anacid containing carbon).

In some embodiments, the concentration of the dilute nitric acid in thefirst cleaning solution is less than about 15 wt %, or more specificallyless than about 10 wt % or about 7 wt %. In some embodiments, the dilutenitric acid concentration can be greater than about 3 wt %. The lowconcentration of nitric acid can effectively remove unreacted nickel,without damaging the transistor device, such as without damaging theunderlying substrates comprising TiN, SiN, and SiO₂.

In some embodiments, the first cleaning step is performed for about 5minutes at a temperature of about 40 C, followed by a water rinse at atemperature of about 25 C (e.g., room temperature) for about 2 minutes.After the first cleaning with dilute nitric acid solution, some of thenickel is removed and some platinum is oxidized, leaving some remainingresidues 168 of nickel, platinum, oxidized platinum, and nickel platinumcompounds. In some embodiments, the first rapid thermal process and/orthe first cleaning are optional.

In FIG. 1E, the substrate is annealed, for example, by a second rapidthermal process, to further reduce the resistance of the nickel platinumsilicide. For example, the high resistive phases of mono-nickel platinumsilicide or di-nickel platinum silicide can further react with siliconto form a low resistive phase of nickel platinum di-silicide. The secondrapid thermal process can involve an anneal in nitrogen ambient, attemperature higher than about 300 C for less than a minute. For example,a rapid thermal process may involve annealing at about 450 C for about30 seconds.

After the second rapid thermal process, the substrate is treated with asecond cleaning solution including dilute aqua regia. The dilute aquaregia can remove remaining nickel, oxidize remaining platinum, dissolveoxidized platinum, and in general, clean the surface of residues ofnickel, platinum and nickel platinum compounds. As discussed above, theaqua regia is preferably diluted, for example, up to 2× more dilute thanprior art aqua regia solution for cleaning nickel platinum residues. Thepresent diluted aqua regia solution can reduce damage to the transistordevices, and can still be effective in removing nickel platinumresidues.

In some embodiments, the dilute aqua regia comprises about 7 wt % nitricacid and about 20 wt % hydrochloric acid. In some embodiments, thedilute aqua regia contains about 14 wt % nitric acid and about 15 wt %hydrochloric acid. In some embodiments, the aqua regia cleaning isperformed for about 5 minutes at temperature about 40 C, followed by awater rinse at 25 C (e.g., room temperature) for about 2 minutes.

In some embodiments, the substrate is treated with a third cleaningsolution to further remove any remaining nickel and platinum residues,such as a cleaning solution containing a mixture of hydrogen peroxideand hydrochloric acid. For example, a solution containing a mixture ofhydrogen peroxide and hydrochloric acid can be applied on the surfacefor less than about 30 minutes, more specifically for less than about 5minutes, at temperature less than or equal about 80 C, and can bebetween about 60 C to about 100 C. In some embodiments, theconcentration of hydrogen peroxide and hydrochloric acid in the mixtureof hydrogen peroxide and hydrochloric acid can be less than or equal toabout 25 wt %.

In some embodiments, the described multiple step cleaning using dilutenitric acid, dilute aqua regia, and a mixture of hydrogen peroxide andhydrochloric acid can effectively remove nickel, platinum andnickel/platinum residues without damage to the transistor device, suchas forming shallow craters devoid of silicide in the Ni(Pt)Si film.Further, the described cleaning process does not attack the differentdevice materials, such as TiN, SiN spacers and SiO₂. The describedcleaning solutions further have additional advantages of prolonged shelflife, for example, up to several months, due to the dilute concentrationof nitric acid and aqua regia. Dilute nitric acid can be easier to usethan concentrated nitric acid, and compared to prior art solutions, suchas SPM. In general, dilute nitric acid does not attack platinum, e.g.,the portion of platinum oxidation states, e.g., Pt⁺ or Pt³⁺, that aredifficult to dissolve in hydrochloric acid can be much smaller than theportion of platinum oxidation state, e.g., Pt⁴⁺, that are more solublein hydrochloric acid.

In some embodiments, the described cleaning solutions can improve nickeland platinum residue removal, especially at high concentrations ofplatinum (e.g., >5 wt % platinum in nickel platinum layer).

FIG. 2 illustrates a process flowchart for cleaning a surface accordingto some embodiments. Operation 200 exposes a surface to a first solutioncomprising a mixture of nitric acid and hydrochloric acid. Operation 210rinses the surface. Operation 220 exposes the surface to a secondsolution containing a mixture of hydrogen peroxide and hydrochloricacid. Operation 230 rinses the surface.

FIG. 3 illustrates a process flowchart for forming a nickel platinumsilicide according to some embodiments. Operation 300 provides asubstrate that includes at least one silicon-containing region.Operation 310 forms a layer including nickel and platinum on thesubstrate. Operation 320 optionally performs a first rapid thermalprocess. Operation 330 optionally exposes the substrate to a firstsolution comprising dilute nitric acid. Operation 340 optionally rinsesthe substrate. Operation 350 performs a second rapid thermal process.Operation 360 exposes the substrate to a second solution comprisingnitric acid and hydrochloric acid. Operation 370 rinses the surface.Operation 380 exposes the substrate to a third solution comprising amixture of hydrogen peroxide and hydrochloric acid. Operation 390 rinsesthe surface.

FIG. 4 illustrates a process flowchart for forming a semiconductordevice according to some embodiments. Operation 400 forms a transistorincluding a gate electrode and a source/drain region over asemiconductor substrate. Operation 410 forms a nickel platinum layerover the semiconductor substrate, covering the gate electrode and/or thesource/drain region. Operation 420 performs a first rapid thermalprocess Operation 430 exposes the substrate to a first solutioncomprising dilute nitric acid. Operation 440 rinses the substrate.Operation 450 performs a second rapid thermal process. Operation 460exposes the substrate to a second solution comprising nitric acid andhydrochloric acid. Operation 470 rinses the surface. Operation 480exposes the substrate to a third solution comprising mixture of ahydrogen peroxide and hydrochloric acid. Operation 490 rinses thesurface.

Although the foregoing examples have been described in some detail forpurposes of clarity of understanding, the invention is not limited tothe details provided. There are many alternative ways of implementingthe invention. The disclosed examples are illustrative and notrestrictive.

What is claimed is:
 1. A method for cleaning nickel and platinumresidues from a surface of a substrate, comprising: exposing the surfaceto a solution comprising nitric acid or sulfuric acid; performing arapid thermal process; exposing the surface to a first solutioncontaining a mixture of nitric acid and hydrochloric acid; and exposingthe surface to a second solution containing hydrochloric acid andhydrogen peroxide.
 2. A method as in claim 1 wherein the temperature ofthe second solution is less than or equal to 80 C.
 3. A method as inclaim 1 wherein the concentration of the hydrogen peroxide in the secondsolution is less than 20 wt %.
 4. A method as in claim 1 wherein theconcentration of the hydrochloric acid in the second solution is lessthan 20 wt %.
 5. A method for forming a nickel platinum silicide, themethod comprising: providing a substrate, the substrate comprising atleast one silicon region; forming a layer comprising nickel and platinumon the substrate; performing a thermal process to form nickel platinumsilicide; exposing the semiconductor substrate to a third solutioncomprising nitric acid or sulfuric acid; performing a second rapidthermal process; removing an un-reacted part of the nickel platinumlayer using a first solution comprising a mixture of nitric acid andhydrochloric acid; and removing platinum residues using a secondsolution containing hydrochloric acid and hydrogen peroxide.
 6. A methodas in claim 5 wherein the temperature of the first solution is less than60 C.
 7. A method as in claim 5 wherein the concentration of the nitricacid in the first solution is less than 10 wt %.
 8. A method as in claim5 wherein the concentration of the hydrochloric acid in the firstsolution is less than 20 wt %.
 9. A method as in claim 5 wherein thetemperature of the second solution is less than or equal to 80 C.
 10. Amethod as in claim 5 wherein the concentration of the hydrogen peroxidein the second solution is less than 20 wt %.
 11. A method as in claim 5wherein the concentration of the hydrochloric acid in the secondsolution is less than 20 wt %.
 12. A method for forming a semiconductordevice, the method comprising: forming a transistor including a gateelectrode and a source/drain diffused layer over a semiconductorsubstrate; forming a nickel platinum layer over the semiconductorsubstrate, covering the gate electrode and the source/drain diffusedlayer; performing a first rapid thermal process; exposing thesemiconductor substrate to a first solution comprising nitric acid;performing a second rapid thermal process; exposing the substrate to asecond solution comprising a mixture of nitric acid and hydrochloricacid; and exposing the substrate to a third solution containinghydrochloric acid and hydrogen peroxide.
 13. A method as in claim 12wherein the first rapid thermal process is performed at temperature lessthan about 380 C.
 14. A method as in claim 12 wherein the concentrationof the nitric acid in the first solution is less than about 10 wt %. 15.A method as in claim 12 wherein the second rapid thermal process isperformed at temperature greater than about 300 C.
 16. A method as inclaim 12 wherein the temperature of the third solution is less than orequal to 80 C.
 17. A method as in claim 12 wherein the concentration ofthe hydrogen peroxide in the third solution is less than 20 wt %.
 18. Amethod as in claim 12 wherein the concentration of the hydrochloric acidin the third solution is less than 20 wt %.